Scientists revive 'giant virus' from Siberian permafrost

Carl Zimmer

Siberia fills the heads of scientists with dreams of resurrection. For millions of years, its tundra has gradually turned to permafrost, entombing animals and other organisms in ice. Some of their remains are exquisitely well-preserved – so well, in fact, that some scientists have nibbled on the meat of woolly mammoths.

Some researchers even hope to find viable mammoth cells that they can use to clone the animals back from extinction. And in 2012, Russian scientists reported coaxing a seed buried in the permafrost for 32,000 years to sprout into a flower.

Now a team of French and Russian researchers has performed a resurrection of a more sinister nature. From Siberian permafrost more than 30,000 years old, they have revived a virus that is new to science.

"To pull out a virus that's 30,000 years old and actually grow it, that's pretty impressive," said Scott O. Rogers of Bowling Green State University who was not involved in the research. "This goes well beyond what anyone else has done."

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The thawed virus, which infects amoebae, is not a threat to humans. But if the new study holds up to scrutiny, it raises the possibility that disease-causing viruses may also be lurking in the permafrost.

The new virus was discovered by a group of researchers led by Chantal Abergel and Jean-Michel Claverie, a wife-and-husband team at Aix-Marseille University in France. Abergel and Claverie are veteran virus hunters, specialising in finding new species of so-called giant viruses.

Familiar viruses are tiny and have few genes. The influenza virus, for example, has 13 genes and is about 100 nanometres across. But giant viruses, which typically infect amoebae, can be 1000 times bigger and have more than 2500 genes.

Scientists had long known that traces of viruses can endure for hundreds or even thousands of years. Researchers have found fragments of smallpox virus genes in 400-year-old Siberian mummies. In Greenland, Rogers of Bowling Green and his colleagues have retrieved genes of plant viruses in ice that froze 140,000 years ago.

But no one had isolated ancient viable viruses that have been able to infect host cells and replicate. Researchers at the Russian Academy of Sciences sent Abergel and Claverie small pieces of permafrost extracted from a Siberian riverbank in 2000.

To search for giant viruses in the samples, the French researchers added bits of the permafrost to colonies of amoebae to see if any viruses in the permafrost could infect them. The amoebae began to die – a sign that something in the permafrost was killing them. When the scientists examined the colonies, they discovered that giant viruses were multiplying inside the amoebae.

The viruses are not quite like anything ever found before. Measuring 1.5 micrometres long, the viruses are 25 per cent bigger than any virus previously found. Their oddly long, narrow shape inspired the scientists to call them pithoviruses – "pithos" referring to ancient Greek earthenware jars.

"Sixty per cent of its gene content doesn't resemble anything on Earth," Abergel said. She and her colleagues suspect pithoviruses may be parasitic survivors of life-forms that were very common early in the history of life.

"Its potential implications for evolutionary theory and health are quite astonishing," said Eske Willerslev, an evolutionary biologist at the University of Copenhagen.

Nonetheless, he said he did not think the scientists had fully ruled out the possibility that their samples had been contaminated with young viruses. "Without it, such claims remain nothing but biological curiosities," Willerslev said.

Abergel and Claverie acknowledged the possibility of contamination. But they noted that they had performed the experiment three times and obtained the same virus from the permafrost each time.

The researchers are trying to confirm their discovery by looking at other permafrost samples. The oldest samples their Russian colleagues sent them are about 3 million years old.

They suspect that if giant viruses can remain viable for 30,000 years or more, other viruses can as well. "I don't see why they wouldn't be able to survive under the same conditions," said Claverie.

It is even possible that some of those viruses could infect humans instead of amoebae. Abergel and Claverie consider it a worrying possibility, given the push into northern Siberia for oil and minerals. Rogers considered the risk of an outbreak of resurrected viruses to be "extremely low", pointing out that scientists have been excavating permafrost and ice for decades without any known infections.